Structural, thermodynamic, and local probe investigations of the honeycomb material Ag3LiMn2 O6

Abstract

© 2019 American Physical Society. Here we present the structural and magnetic properties of the new honeycomb material Ag3LiMn2O6. The system Ag[Li1/3Mn2/3]O2 belongs to a quaternary 3R-delafossite family and crystallizes in a monoclinic symmetry with space group C2/m, and the magnetic Mn4+(S=3/2) ions form a honeycomb network in the ab plane. An anomaly around 50 K and the presence of antiferromagnetic (AFM) coupling (Curie-Weiss temperature θCW∼-51K) were inferred from our magnetic susceptibility data. The magnetic specific heat clearly manifests the onset of magnetic ordering in the vicinity of 48 K, and the recovered magnetic entropy, above the ordering temperature, falls short of the expected value, implying the presence of short-range magnetic correlations. An asymmetric Bragg peak (characteristic of two-dimensional order), seen in neutron diffraction, gains intensity even above the ordering temperature, thus showing the existence of short-range spin correlations. Our electron spin resonance (ESR) experiments corroborate the bulk magnetic data. Additionally, the ESR line broadening upon approaching the ordering temperature TN could be described in terms of a Berezinskii-Kosterlitz-Thouless scenario with TKT=40(1)K. The Li7 NMR line shift probed as a function of temperature tracks the static susceptibility (Kiso) of magnetically coupled Mn4+ ions. The Li7 spin-lattice relaxation rate (1/T1) exhibits a sharp decrease below about 50 K. A critical divergence is absent at the ordering temperature perhaps because of the filtering out of the antiferromagnetic fluctuations at the Li site, i.e., at the centers of the hexagons in the honeycomb network. Combining our bulk and local probe measurements, we establish the presence of an ordered ground state for the honeycomb system Ag3LiMn2O6. Our ab initio electronic structure calculations suggest that in the ab plane, the nearest-neighbor (NN) exchange interaction is strong and AFM, while the next-NN and the third-NN exchange interactions are FM and AFM, respectively. The interplanar exchange interaction is found to be relatively small. In the absence of any frustration the system is expected to exhibit long-range, AFM order, in agreement with experiment.